Internal combustion engines may produce NOx as a byproduct of combustion of air and fuel. NOx is a regulated vehicle exhaust emission, and as such, it may be desirable to ascertain whether or not an engine is producing an expected amount of NOx. Larger amounts of NOx may be produced at higher engine speeds and loads where exhaust gases combust and produce higher temperatures and pressures. NOx formation in engine cylinders may be reduced via reducing pressure and temperature within a cylinder. NOx may be reduced via reducing pressure in a cylinder, but reducing cylinder pressure may also reduce engine output. However, temperature in a cylinder may be reduced without engine power loss at low and medium engine loads via combining exhaust gases with fresh charge (e.g., air and fuel) in the cylinder. Exhaust gases from a previous combustion event may be retained and/or drawn into a cylinder via simultaneously opening intake and exhaust valves, which may be referred to as positive intake and exhaust valve overlap. Exhaust gas recirculation occurring within a cylinder via valve timing may be referred to as IEGR. Nevertheless, if intake valve and exhaust valve overlap is too long or too short, engine misfire or elevated NOx levels may result. Therefore, it may be desirable to determine whether or not a desired amount of IEGR is provided.
The inventor herein has recognized the above-mentioned disadvantages of too much or too little IEGR and has developed a method, comprising: rotating an engine that is not combusting an air-fuel mixture and providing a difference between a sensor output when an EGR system is commanded to a baseline position and the sensor output when the EGR system is commanded to a non-baseline position in response to a request to diagnose the EGR system; and adjusting output of the engine when the engine is combusting air and fuel responsive to the difference.
By rotating an engine unfueled and sensing flow through the engine, it may be possible to provide the technical result of diagnosing operation of an IEGR system without increasing engine NOx output. In particular, an IEGR system may adjust intake and exhaust valve timing such that at base positions, engine air flow though the unfueled engine is greater than when intake and exhaust valve timing is adjusted to provide increased IEGR, if the IEGR system is operating as is desired. However, if the IEGR system is not operating as is desired, air flow through the unfueled engine having intake and exhaust valve timing adjusted to provide increased IEGR may be greater than is desired. The higher air flow rate through the engine may be indicative of IEGR system degradation.
The present description may provide several advantages. Specifically, the approach may diagnose operation of an IEGR system without increasing engine NOx output. Further, the approach may perform IEGR diagnostics using sensors other than sensors that may typically be used to diagnose an IEGR system so that independent verification of the IEGR system may be provided. Further still, the approach may vehicle be performed without degrading vehicle drivability when the IEGR system is diagnosed.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.